A stable-labeled internal standard suitable for quantitation of 1,25-Dihydroxyvitamin D3 levels by LC-MS/MS. 1,25-Dihydroxyvitamin D3, also known as calcitriol, is a hormonally active metabolite of vitamin D responsible for regulating calcium. Measurement of 1,25-Dihydroxyvitamin D3 is used to...